1. Field of the Invention
This invention relates to the pyrometallurgical production of ferrous sulphide.
2. Brief Description of the Prior Art
Ferrous sulphide has long been produced by charging a furnace with a suitable source of sulphur, which might be sulphur itself or an iron disulphide such as iron pyrites, and a suitable source of metallic iron, forming a molten reaction mixture of the source of sulphur and the source of metallic iron by operation of the furnace, allowing reaction between the sulphur or the iron disulphide and the metallic iron in the reaction mixture to occur, tapping the furnace of the resulting molten ferrous sulphide product and subsequently disposing of unwanted slag produced.
Furnaces directly heated by the combustion of a fuel, such as fuel oil, were originally used and are still being used in processes as above described for the production of ferrous sulphide despite disadvantages associated with this type of furnace. Residual oxygen present in the combustion gases, as well as oxygen or oxides present in the furnace charge, react with the sulphur content of the raw materials to an appreciable extent to form sulphur dioxide the allowed emission levels of which are strictly controlled by regulation in most industrial countries. The necessity of controlling sulphur dioxide emissions has led to need to scrub the stack gases emitted from conventional direct-fired ferrous sulphide production furnaces and even when the sulphur values so removed are recovered in usable form this does not represent an economic utilisation of the sulphur. A further consequence of the presence of oxygen in the furnace combustion gases may be the formation of ferrous oxide the presence of which, as noted in British Patent Specification No. 1281284, can cause wear of alumina or silica furnace linings. Although at low residual oxygen concentrations this effect may not be marked it nevertheless represents a potential contributory factor to furnace lining wear.
As long ago as the year 1916 the problems associated with the use of direct-fired furnaces in the production of ferrous sulphide were described in U.S. patent specification No. 1169093 and a method of avoiding them proposed. According to that method the source of sulphur to be used was an iron pyrites containing sufficient siliceous impurities to form a slag blanket over the melt. The effect of the slag was to reduce losses of sulphur, presumably in the form of sulphur dioxide formed by reaction with oxygen or oxides present in the furnace charge, from the melt surface. The furnace to be used was an electric furnace of the arc type. The rapid melting of the charge in such a furnace caused the immediate formation of slag so that it could be maintained as a protective blanket from the inception to the end of the process. If an iron pyrites containing sufficient impurities to attain a slag blanket having the required thickness was unavailable the deficiency could, to a degree, be supplied by the addition of sand or other slag-forming substances to the charge. However the need to use low grade iron pyrites as a raw material decreases cost efficiency per unit of ferrous sulphide product due to the greater quantity of raw material to be heated.